1. Field of the Invention
This invention relates generally to a disk record medium, a method for recording and/or reproducing information onto and from the same, and a recording/reproducing apparatus for carrying out the method. More particularly, the invention relates to a recording/reproducing method and a recording/reproducing apparatus suitable for realizing a mass storage disk file system.
2. Description of the Related Art
Currently, recording of information in a record medium in the form of an optical disk is accomplished by providing a spiral track or concentric tracks on the optical disk and by forming on the track or tracks pits or domains such as physically varied regions, which can be detected by an optical means or a magnetic means, in association with the information to be recorded. For reproducing the recorded information, an object track is selected from many tracks juxtaposed in a radial direction of the disk, and the physically varied regions are detected.
Although it is actually continuous a spiral track, appears as if a plurality of tracks are arranged radially on the disk. It is therefore regarded, in the present invention, that plural tracks are provided on a disk having a spiral track.
Two methods are hitherto known to record and/or reproduce information in and/or from such a disk record medium. One of the two methods is a CAV (Constant Angular Velocity) method in which recording and/or reproducing is performed while the disk is rotating at a constant angular velocity; and the other is a CLV (Constant Linear Velocity) method in which recording and/or reproducing takes place while the disk is rotating at a constant linear velocity.
According to the former method, although recording and/or reproducing can be performed stably, the record density is low, and the rate of error occurrence or the error frequency during signal reproduction varies between the inner and outer tracks of the disk, that is, there is a difference in quality of signals. According to the latter method, on the other hand, although the record density can be high, the access speed is slow because the rate of rotation of the disk is varied depending on the radial location of a record position to be accessed.
To this end, a solution has been proposed by, for example, Japanese Patent Laid-Open Publication No. 61-131236 (1986). This Japanese publication discloses a method in which the record pit or domain lengths of both the inner and outer tracks are kept constant throughout the entire record region by changing the recording clock frequency according to the linear velocity of the track on the record medium rotating at a constant angular velocity.
This prior art is also exemplified by Japanese Patent Laid-Open Publications Nos. 60-177404 (1985) and 60-117448 (1985).
As mentioned above, in a recording/reproducing apparatus employing a disk record medium which is rotated at a constant angular velocity, the record pit or domain length can be constant at both the inner and outer tracks in the record area by increasing the recording/reproducing frequency at the outer track depending on the linear velocity of the record position. Thus, an increased record capacity of the disk is to be expected.
These prior art references, however, are totally silent about either the record pit or domain length on a disk record medium, or the relation to the recording method. These prior art references are also silent about how the pit or domain length varies from the inner track to the outer track.
It has come to light, from the present inventors' studies, that these factors are important to realize stable reproduction of information. Particularly in a so-called pit-edge record method or a mark-length modulation method in which the front and rear edges of a pit or record domain formed when a signal is defined during recording, it has also turned out that the extent of displacement of the edge-detection position is increased because of possible variation of the pit or domain length, thus deteriorating the detected information.
More specific discussion will now be provided concerning clocks. A clock encounters fluctuation caused by the associated circuits. Assuming that the extent of the variation due to the fluctuation is substantially constant irrespective of the frequency, the higher the frequency is, the greater the influence of the variation relative to the clock width.
Secondly the relation between the provision of a pit and the linear velocity will be discussed. The present inventors conducted an experiment, the results of which indicate that the rising and lowering temperature gradients, which may be caused during the forming of the pit according to the characteristics of a record medium, depend on the linear velocity. The present inventors further conducted another experiment, in which pits were formed at a variety of linear velocities and the formed pits were then detected by using a detecting window. The second experiment shows that the higher the linear velocity is, the more the displacement extent of the position of an edge detection signal relative to the window margin was increased. In other words, as shown in FIG. 9 of the accompanying drawings, if the record clock frequency is varied so as to provide the same record pit length on both the inner and outer track sides of the record area, then the ratio .DELTA..PHI./W of the extent of displacement .DELTA..PHI. to the window margin W shows a tendency to rise from the inner track toward the outer track.
Thus, as the record position is nearer to the outer track, the the ratio .DELTA..PHI./W is increased. Consequently, in the case the recording is carried out with the pit lengths constant, the displacement of the detection signal appearing position with respect to the detecting window is increased to such an extent that a detection signal cannot be received in the detecting window. As a result, a reading error occurs which deteriorates the quality of the signal so that stable reproducing cannot be realized.
Furthermore, changing the clock for each and every track requires both a means for generating many different clocks so as to meet with many tracks and a means for switching clocks for every track, thus making the circuits complex. Additionally, a special area is needed for the clock switching position of every track, thus requiring a complex control and reducing the entire record area.